The invention relates to a vacuum measuring cell with membrane.
Total vacuum measuring cells are utilized for measuring the total pressure in vacuum chambers. The vacuum measuring cells are employed in monitoring various vacuum processes. Typical vacuum processes are here surface working processes, such as for example coating methods and etching methods. Such methods operate, for example, in pressure ranges of 10−5 mbar to 100 mbar. It is customary to measure such pressure ranges with so-called Pirani vacuum measuring cells or with so-called membrane vacuum measuring cells. One problem encountered herein is that the process gas, however also residual gas components, can contaminate the vacuum sensor during the process. As a consequence, imprecise or erroneous measurements or pressure indications can herein be generated. The vacuum measuring cells exhibit drift behavior commensurate with the time during which they are exposed to the process, which cannot always be eliminated or restored again by cleaning the vacuum cell or its surroundings. Membrane measuring cells are especially sensitive with respect to possible contaminations. In such membrane measuring cells a thin membrane is deflected as a function of the obtaining pressure which is to be measured. The deflection of this membrane is measured and serves as the measurement for the vacuum pressure to be measured. In the case of capacitive membrane measuring cells the deflection of the membrane is measured via the variation of the capacity between membrane and the solid body. In optical membrane measuring cells this deflection is acquired using optical methods, for example with interferometric methods. In order to be able to measure such pressure ranges with high sensitivity, the membranes must herein be implemented such that they are very thin, for example in the range of 40 to 760 μm. Contaminations of this thin membrane, for example by gas and/or particles, whereby even an accumulation layer can form, can lead to tension and/or compression stresses on the membrane, which additionally affects the deformation of the membrane and, as a consequence, leads to error measurements, for example in the absolute values to be measured, or to undesirable drift behavior over time. In addition, herein the resolution, and therewith the precision, of the measuring cell is decreased and, on the other hand, the reproducibility of the measuring results is not ensured.
In order to decrease such contaminations, until now a planar screen, technically also referred to as a baffle, has been utilized as is shown by example of a capacitive membrane vacuum measuring cell in FIG. 1. The vacuum measuring cell 15 is comprised of a first planar and round housing part 1 and a second planar and round housing part 4, wherein between these two housing parts a membrane 2 is connected with the formation of sealing via seals 3, for example a glass solder, such that between the membrane and each of the two housing parts one hollow volume 9, 10 is formed. The one hollow volume forms a reference vacuum volume 10 which communicates with a getter volume 12 via a connection line 13. In the getter volume 12 is located a getter 11 for the reliable maintenance of a reference vacuum. Opposite the reference vacuum volume 10 on the other side of membrane 2 the measuring vacuum volume 9 is formed which communicates via an outlet opening 16 with the baffle housing 6 in which a baffle 7 is located, the baffle housing being appropriately connected with the vacuum measuring cell 15 via, for example, a connection fitting 5. On the baffle housing 6 is disposed a connection flange 8 with a connection opening 22 which can be connected with the vacuum process chamber to be measured. The connection opening 22 is disposed such that the baffle 7 prevents the direct line-of-sight with respect to the outlet opening 16 of the vacuum measuring cell: herewith the baffle 7 is intended to unfold its protective effect thereby that the undesirable gases or particles are condensed on the baffle surface such that they do not reach the vacuum measuring cell. In the technical literature this baffle is often also referred to as a plasma shield. In processes containing reactive gases such are preferably intended to condense on the baffle. Hereby the sensor drift is to be reduced and thereby the service life of the measuring cell is to be increased. Although this planar baffle improves the service life of the measuring cell, it cannot be prevented that there is still a significant fraction of particles around the baffle, for example also enhanced through scattering processes, reaching the measuring membrane and here falsify the measurement.